Radio Frequency Switch with Low Oxide Stress

ABSTRACT

A RF switch circuit includes a voltage divider circuit and a semiconductor device. The semiconductor device has an activated state and a deactivated state. The voltage divider circuit has an input terminal connected to a first line and an output terminal connected to a second line. The first line is connected to a power source. A gate terminal of the semiconductor device is connected to the second line. In the activated state, a source terminal and a drain terminal of the semiconductor device are each selectively connected to ground. In the deactivated state, the source terminal and the drain terminal of the semiconductor device are each selectively connected to the power source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to and claims the benefit of U.S. ProvisionalApplication No. 62/029,344, filed Jul. 25, 2014 and entitled “RADIOFREQUENCY SWITCH WITH LOW OXIDE STRESS,” the entirety of the disclosureof which is wholly incorporated by reference herein.

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

Not Applicable

BACKGROUND 1. Technical Field

The present disclosure generally relates to the field of electronics.More particularly, the present disclosure relates to radio frequency(RF) switches with low oxide stress.

2. Related Art

High frequency switches are used in wireless communications and radarsystems for switching between the transmit (Tx) and receive (Rx) paths,for routing signals to the different blocks in multi-band/standardphones, for RF signal routing in phase shifters used in phased arrayantennas, etc. The communication frequencies cover a broad range fromthe below-a-megahertz AM band, to the commercial FM band (88-108 MHz),to various military hand-held radio transceivers up to approximately 400MHz, and to the cellular frequencies of around 900 MHz and 2.4 GHz.State-of-the-art switch technology uses solid-state semiconductordevices for their integration compatibility and relatively lowermanufacturing cost attributed to batch fabrication. With theconventional approach, the wide spectrum of communication frequenciescalls for different switch technologies for different frequencyapplications.

Wireless communication devices may be composed of a transmit chain and areceive chain, with the antenna and the transceiver circuit being a partof both the transmit chain and receive chain. The transmit chain mayadditionally include a power amplifier for increasing the output powerof the generated RF signal from the transceiver, while the receive chainmay include a low-noise amplifier for boosting the weak received signalso that information can be accurately and reliably extracted therefrom.

The low-noise amplifier and the power amplifier may together consist ofa front-end module or front-end circuit, which also includes an RFswitch circuit that selectively interconnects the power amplifier andthe low-noise amplifier to the antenna. The connection to the antenna isswitchable between the receive chain circuitry (i.e., the low-noiseamplifier and the receiver) and the transmit chain circuitry (i.e., thepower amplifier and the transmitter). In time domain duplex (TTD)communications systems where a single antenna is used for bothtransmission and reception, switching between the receive chain and thetransmit chain occurs rapidly many times throughout a typicalcommunications session.

The RF switches and the amplifier circuits of the front-end module aretypically manufactured as an integrated circuit. In high-powerapplications such as GSM (Global System for Mobile communications)handsets, WLAN (wireless local area networking) client interface devicesand infrastructure devices, the ICs are typically manufactured with aGaAs (gallium arsenide) semiconductor substrate. The SOI(silicon-on-insulator) process has also found use in RF switch circuitapplications. Good insertion loss and isolation are possible with bothGaAs and SOI processes, but manufacturing costs tend to be higher incomparison to more conventional semiconductor technologies, such as theCMOS (Complementary Metal Oxide Semiconductor) process. There have beenattempts to implement RF switches in the CMOS process, but only lowpower devices have been realized. This is, in part, due to the parasiticcapacitance of transistors and low-resistivity substrates of bulksemiconductor wafers used in the CMOS process. Accordingly,high-isolation high-linearity CMOS switches at large RF signal levelshave been difficult to achieve.

Several electrical parameters are associated with the performance of RFswitch designs, but four are considered to be of fundamental importanceto the designer because of their strong interdependence: insertion loss,isolation, switching time and power handling. Insertion loss refers topower loss in the RF switch, and is expressed in dB. It is defined byPout-Pin (dB), where Pin is the input power applied to the RF switch,and Pout is the power at the output port of the RF switch. Thus, a goalof RF switches is to minimize insertion loss. Isolation is a measure ofhow effectively a switch is turned off. It refers to the measure of thesignal attenuation between the active signal port and the inactivesignal port. The main contributing factors include capacitive couplingand surface leakage. Thus, a goal of RF switches is to maximizeisolation, which minimizes signal leakage. Return loss generally refersto the amount by which the undesired return (or reflected) transmitsignal is attenuated. It refers to the measure of input and/or outputmatching conditions, and is expressed in dB. Linearity, or powerhandling capability, is the capability of the RF switch to minimizedistortion at high power output levels and is expressed in dBm. It istypically represented by the 1 dB compression point (P1 dB), or thepoint at which insertion loss is degraded by 1 dB. Thus, a goal of RFswitches is to maximize linearity. Switching time is the period of timea switch needs for changing state from “ON” to “OFF” and “OFF” to “ON.”This period can range from several microseconds in high-power switchesto a few nanoseconds in low-power, high-speed devices.

RF switches are designed to generate as little harmonic distortion aspossible. Governmental standards also restrict the output of spuriousemissions including those from harmonic distortion to either −70 dBc or43+10 log(P). Conventional front end circuits, including the RF switch,may be fabricated on a bulk CMOS (complementary metal oxidesemiconductor) substrate. However, there is a performance tradeoffbetween insertion loss and harmonic distortion under large signaloperation. Furthermore, because of low mobility, low breakdown voltage,and high substrate conductivity associated with CMOS devices, an RFswitch with low insertion loss, high isolation, wide bandwidth, andlinearity is difficult to produce.

The main performance characteristics of an RF switch are the insertionloss in the ON-state, the isolation in the OFF-state, the return loss inboth states, the power consumption, bandwidth, power handling capabilityand the linearity. RF switching is presently realized for the greaterpart with PIN diode and GaAs MESFET, HEMT, JFET or silicon SOI basedsemiconductor switches. RF switches are used in a variety ofapplications, such as traditional single pole multi throw switches(e.g., for general purpose switching, band/mode switching and antennadiversity applications), double pole double throw (DPDT) switches andantenna switch modules (ASM). Highly linear band or mode switches arevital for multi-mode and multi-band architectures as mobile phone radioscan easily operate in 14 bands or more. Typically single-pole,double-throw (SPDT) or single-pole, three-throw (SP3T) switches are useddepending on the number of bands supported.

There is a continuing need in the art for improved RF switches, whethersingle pole-double throw, single pole-triple throw, dual port-dualthrow, or any other switch type. There is a need to provide a circuitarrangement with a RF switch circuit having faster switching time. Thereis a need for RF switches that can be implemented on CMOS substrates orany other semiconductor technology while minimizing insertion loss andmaximizing isolation and linearity. There is a need for silicon RFswitches with high power handling capabilities.

BRIEF SUMMARY

The present disclosure is directed to RF switch circuits and methods forconfiguring and operating semiconductor switches for processing of radiofrequency (RF) signals. The presently-disclosed techniques allow the useof standard low-cost silicon technologies (e.g., CMOS) forradio-frequency integrated circuits (RFICs) manufacturing.

According to an aspect of the present disclosure, there is a circuitarrangement comprising a RF switch circuit. The RF switch circuitincludes a voltage divider circuit and a semiconductor device. Thesemiconductor device has an activated state and a deactivated state. Thevoltage divider circuit has an input terminal connected to a first lineand an output terminal connected to a second line. The first line isconnected to a power source. A gate terminal of the semiconductor deviceis connected to the second line. In the activated state, a sourceterminal and a drain terminal of the semiconductor device are eachselectively connected to ground. In the deactivated state, the sourceterminal and the drain terminal of the semiconductor device are eachselectively connected to the power source.

According to another aspect of the present disclosure, there is a methodfor configuring and operating a semiconductor switch for processing ofRF signals. The method includes applying an input voltage over a firstline to a voltage divider circuit, and applying an output voltage of thevoltage divider over a second line to a gate terminal of a triple-wellNMOS transistor. The output voltage of the voltage divider circuit isalways above a threshold of the triple-well NMOS transistor to guaranteeswitch function.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects and features of the presently-disclosed RF switch circuits andmethods for configuring and operating semiconductor switches forprocessing of RF signals will become apparent to those of ordinary skillin the art when descriptions of various embodiments thereof are readwith reference to the accompanying drawings, of which:

FIG. 1 is a circuit diagram schematically depicting a typical controlvoltage distribution in a triple-well NMOS transistor, used as an RFswitch, for the ON-state;

FIG. 2 is a circuit diagram schematically depicting a typical controlvoltage distribution in the triple-well NMOS transistor of FIG. 1 forthe OFF-state;

FIG. 3 is a circuit diagram schematically depicting a control voltagedistribution in a triple-well NMOS transistor, used as an RF switch, forthe ON state, in accordance with an embodiment of the presentdisclosure;

FIG. 4 is a circuit diagram schematically depicting a control voltagedistribution in the triple-well NMOS transistor of FIG. 3 for the OFFstate, in accordance with an embodiment of the present disclosure;

FIG. 5 is a circuit diagram schematically depicting the power handlingcapability of the triple-well NMOS transistor of FIG. 4 in accordancewith an embodiment of the present disclosure; and

FIG. 6 is a flowchart illustrating a method for configuring andoperating a semiconductor switch for processing of RF signals inaccordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

Hereinafter, embodiments of an RF switch circuit and embodiments of amethod for configuring and operating semiconductor switches forprocessing of RF signals are described with reference to theaccompanying drawings. Like reference numerals may refer to similar oridentical elements throughout the description of the figures.

This description may use the phrases “in an embodiment,” “inembodiments,” “in some embodiments,” or “in other embodiments,” whichmay each refer to one or more of the same or different embodiments inaccordance with the present disclosure.

Various embodiments of the present disclosure provide a method forconfiguring and operating semiconductor switches for processing of RFsignals wherein a relatively low-voltage transistor (e.g., 3.3Vtransistor) can be used for high-voltage applications (e.g., 5V).Various embodiments of the presently-disclosed method for configuringand operating semiconductor switches for processing of RF signals mayprovide faster switching time, handle greater power, and keep the devicereliable at higher voltages than usual. The presently-disclosed methodsfor configuring and operating semiconductor switches for processing ofRF signals may reduce voltage stress level over gate oxide to half whilemaintaining the same level of power handling capability.

Referring now to FIG. 1, there is shown a circuit diagram of a circuit100, which depicts a typical control voltage distribution in atriple-well NMOS transistor 116, used as an RF switch, for the ON-state(also referred to herein as the “activated state”).

The NMOS transistor 116 has a gate terminal connected via a resistor R5to an input line 118. The input line 118 is connected to a power source,e.g., a battery (not shown). The drain terminal is connected to the node162. The node 162 is connected via a resistor R6 to ground (GND). Thesource terminal of the NMOS transistor 116 is connected to the node 164.The node 164 is connected via a resistor R4 to ground. A capacitor 122is coupled between the node 164 and an RF signal input line 102. Thecapacitor 122 may be a DC blocking capacitor. The circuit 100 alsoincludes parasitic diodes D1, D2 and D3 associated with semiconductorsubstrate.

As shown in FIG. 1, when the triple-well NMOS transistor 116 is in theactivated state, the gate voltage is at VDD, and the drain and sourceterminals are at ground. In an illustrative non-limiting example whereVDD is 5V, the oxide DC stress is Vgs=Vds=5V. This is beyond the limitof, for example, 3.3V devices.

In general, the use of triple-well NMOS transistors reduces circuit andlayout complexity. Significantly, having individual NMOS transistorsbuilt in separate p-wells in the triple-well scheme allows the variablecontrol of substrate bias. Triple well technology enables application ofsubstrate voltage separately to each transistor and allows the thresholdvoltage of the transistors to be altered dynamically.

The triple-well technology comprises a buried n-well layer that isolatesthe p-well from the p-type substrate. The deep n-well in the triple welltechnology isolates the p-type substrate from the p-well, thus reducingsubstrate noise coupling. Triple wells can be fabricated in a number ofways. For example, in a p-type substrate having a doping density of1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³, an n-type implant can be made. Arsenic atomsare typically implanted as opposed to phosphorus, because arsenicdiffuses slower than phosphorus and has a better lattice match withsilicon. The n-type implant must be deep enough to prevent it frominfluencing the device behavior of the NMOSFETs that are fabricated inthe p-well. Surrounding this buried n-well, using a separate mask,n-type implants are made. This surrounds the NMOS device or a group ofdevices in the same island. The n-type implant must contact the buriedn-type implant, and there should be no p-type layer between the n-typeimplant and the p-type substrate. Following these steps, a p-type layeris grown epitaxially. This forms the p-well. The MOSFETs are builtconventionally in the wells thus formed. If each NMOSFET is built in aseparate p-well that is surrounded by the n-type implants, the thresholdvoltage of each transistor can be controlled individually by adjustingthe bias on the p-well.

An alternate method of fabricating triple wells begins by growing ap-layer epitaxially, followed by implanting the n-well. Boron isimplanted at a dose between 1×10¹⁵ cm⁻² and 5×10¹⁵ cm⁻² at an energybetween 2 keV and 10 keV, into a p-type substrate having a dopingdensity between 1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³. A Ge implant preceding thisimparts a degree of amorphousness to the substrate, allowing higherdopant concentrations. A mono-crystalline p-type layer is grownepitaxially. This layer is lightly doped, having doping of the sameorder of magnitude as the substrate. An n-type ion implantation isperformed next to form the n-well, and a p-type implantation isperformed for the p-well. In the p-well, arsenic is implanted to form aburied n-layer. The n-type buried layer could be made discontinuous byleaving a gap in the layer. This allows a contact between the p-well andthe p-type buried implant. The p+ contact to the p-well is made exactlyabove the gap in the n-layer. The transistors and the well contacts aremade using the conventional process. Leaving the gap in the deep n-layerallows the formation of a low-resistance path between the source of thePMOSFET built in the n-well and the p-well through the n-well. Thishelps in reducing the potential drop along this path, and the chances oflatchup are minimized, latchup being a major reliability concern in bulkCMOS technology.

As shown in FIGS. 1 through 5, the resistivity of the p-type substrateis indicated by the resistor R1. The resistivity of the deep n-well isindicated by the resistor R2. The resistivity of the bulk semiconductoris indicated by the resistor R3.

Referring now to FIG. 2, there is shown the circuit 100 of FIG. 1,depicting a typical control voltage distribution in the triple-well NMOStransistor 116 for the OFF-state (also referred to herein as the“deactivated state”). As shown in FIG. 2, when the triple-well NMOStransistor 116 is in the deactivated state, the gate voltage is atground (GND), and the drain and source terminals are at VDD. In theillustrative non-limiting example where VDD is 5V, the oxide DC stressis Vgs=Vds=5V. This is beyond the limit of, for example, 3.3V devices.

Referring now to FIGS. 3 through 5, there is shown a circuit diagram ofa circuit 300 in accordance with an embodiment of the present disclosurethat includes the circuit 100 having the triple-well NMOS transistor116. FIG. 3 shows a voltage distribution in the triple-well NMOStransistor 116 for the activated state. FIG. 4 shows a control voltagedistribution in the triple-well NMOS transistor 116 for the deactivatedstate.

The circuit 300 includes a voltage divider circuit 390, which may be atwo-resistor voltage divider. It is to be understood that any suitablevoltage divider arrangement may be utilized. As shown in FIGS. 3 through5, the circuit 300 includes a line 318 to an input terminal of thevoltage divider circuit 390. The line 318 is connected to a powersource, e.g., a battery (not shown).

The circuit 300 includes a line 118 connected between an output terminalof the voltage divider circuit 390 and the gate terminal of thetriple-well NMOS transistor 116. As shown in FIG. 3, when thetriple-well NMOS transistor 116 is in the activated state, the gatevoltage is at VDD/2, and the drain and source terminals are at ground.As shown in FIG. 4, when the triple-well NMOS transistor 116 is in thedeactivated state, the gate voltage is at VDD/2, and the drain andsource terminals are at VDD. In other words gate terminal voltage may bekept at constant DC voltage in both ON and OFF state of the switch. Asmentioned above voltage divider may be resistive with very low currentthus having high resistance values. However turn-on and turn-off time ofthe switch is not compromised.

In the illustrative non-limiting example where VDD is 5V, the oxide DCstress is Vgs=Vds=2.5V. This is within the limit of, for example, 3.3Vdevices. In accordance with the arrangement shown in circuit 300, VDD/2is always above the threshold (<1V) of the triple-well NMOS transistor116.

FIG. 5 shows the power handling capability of the triple-well NMOStransistor 116 in accordance with an embodiment of the presentdisclosure. The OFF-state sets the RF power handling limit because ofparasitic diode (diodes D1, D2, D3) clipping at certain level of inducedRF voltage. Vbulk=VRF/2 at node 160 because of capacitor divider(diodes' parasitic capacitance).

Diode clips when Vbulk−Vd (or Vs)>Vth of diode (i.e., 0.7V), andVd=Vs=Vdd when switch is off. As a result, the power handling can beexpressed as set forth, below, in equation 1.

$\begin{matrix}\frac{\left. \left\lbrack {\left( {{VDD} + 0.7} \right) \times 2} \right) \right\rbrack^{2}}{2 \times 50} & (1)\end{matrix}$

Thus, using equation 1, when VDD equals 3.6V, power handling=29 dBm.

FIG. 6 shows a flowchart illustrating a method 600 for configuring andoperating a semiconductor switch for processing of RF signals inaccordance with an embodiment of the present disclosure. At block 610,an input voltage VDD is applied over a first line 318 to a voltagedivider circuit 390.

At block 620, an output voltage of the voltage divider circuit 390 isapplied over a second line 118 to a gate terminal of a triple-well NMOStransistor 116. The output voltage of the voltage divider circuit 390 isalways above a threshold of the triple-well NMOS transistor 116 toguarantee switch function.

In some embodiments, when the triple-well NMOS transistor 116 is in adeactivated state, a source terminal and a drain terminal of thetriple-well NMOS transistor 116 are each selectively connected to apower source, the power source being connected to the voltage dividercircuit 390 over the first line 318.

In some embodiments, when the triple-well NMOS transistor 116 is in anactivated state, a source terminal and a drain terminal of thetriple-well NMOS transistor 116 are each selectively connected toground.

Although embodiments have been described in detail with reference to theaccompanying drawings for the purpose of illustration and description,it is to be understood that the disclosed processes and apparatus arenot to be construed as limited thereby. It will be apparent to those ofordinary skill in the art that various modifications to the foregoingembodiments may be made without departing from the scope of thedisclosure. Further, the various features of the embodiments disclosedherein can be used alone, or in varying combinations with each other andare not intended to be limited to the specific combination describedherein. Thus, the scope of the claims is not to be limited by theillustrated embodiments.

What is claimed is:
 1. A circuit arrangement comprising a radiofrequency (RF) switch circuit, the RF switch circuit comprising: avoltage divider circuit having an input terminal connected to a firstline and an output terminal connected to a second line, wherein thefirst line is connected to a power source; a semiconductor device havingan activated state and a deactivated state; wherein a gate terminal ofthe semiconductor device is connected to the second line; wherein, inthe activated state, a source terminal and a drain terminal of thesemiconductor device are each selectively connected to ground; andwherein, in the deactivated state, the source terminal and the drainterminal of the semiconductor device are each selectively connected tothe power source.
 2. The circuit arrangement of claim 1, wherein thesemiconductor device is a triple-well NMOS transistor.
 3. The circuitarrangement of claim 1, wherein the source terminal of the semiconductordevice is connected to an RF signal input line.
 4. The circuitarrangement of claim 1, wherein the drain terminal of the semiconductordevice is connected to an RF signal output line.
 5. A method forconfiguring and operating a semiconductor switch for processing of RFsignals, comprising: applying an input voltage over a first line to avoltage divider circuit; applying an output voltage of the voltagedivider over a second line to a gate terminal of a triple-well NMOStransistor, the output voltage of the voltage divider circuit alwaysbeing above a threshold of the triple-well NMOS transistor to guaranteeswitch function.
 6. The method of claim 5, wherein, when the triple-wellNMOS transistor is in a deactivated state, a source terminal and a drainterminal of the triple-well NMOS transistor are each selectivelyconnected to a power source, the power source being connected to thevoltage divider circuit over the first line.
 7. The method of claim 5,wherein, when the triple-well NMOS transistor is in an activated state,a source terminal and a drain terminal of the triple-well NMOStransistor are each selectively connected to ground.